This invention relates to a mechanism for indexing to a defined rotary position. It can be used in an arm for holding a probe in a machine tool.
It is known to mount a toolsetting probe in a machine tool. The probe is mounted to the bed or table of the machine tool, such that a cutting tool can be moved into contact with it. This enables the location of the cutting tip of the tool to be established, in order to set offsets associated with the tool for use by the numerical control of the machine.
In many machines, the toolsetting probe would get in the way if permanently mounted in the required position. Consequently, it is known to mount it on a movable arm, which in turn is mounted to the bed of the machine.
For example, our U.S. Pat. No. 5,446,970 shows such an arm, which is rotatable between an operative position and an inoperative position. The operative position is defined by elements which form a stop against further rotary motion beyond the operative position. These elements are arranged as a kinematic support, so that the operative position is defined in a precisely repeatable manner. A relatively complex arrangement of tension springs provides a biasing force acting in a circumferential direction, biasing the elements of the stop together.
One aspect of the present invention provides a rotary mechanism comprising:
a fixed member;
a rotatable member which is rotatable about an axis relative to the fixed member;
a stop between the fixed and rotatable members, which defines an index position in the rotation of the rotatable member and prevents further rotation thereof past the index position;
the rotatable member being biased by a rotational force into engagement with the fixed member at said stop when the rotatable member is in the index position;
wherein the rotational force is derived from a biasing device acting in the axial direction.
The axial biasing device may be a spring, such as a planar spring.
The rotational force may be derived from the axial force by a detent mechanism, in which a detent element is axially biased against an edge or surface which extends generally radially.
Preferably the stop comprises at least one element in each of three circumferentially spaced locations on the rotatable member; and at least one element in each of three corresponding circumferentially spaced locations on the fixed member; said elements on the fixed and rotatable members at each said location being urged together by the rotational force.
Preferably said elements on the fixed and rotatable members co-operate with each other to constrain the degrees of freedom of the rotatable member kinematically, thereby precisely defining the index position of the rotatable member.
One or more pairs of the elements on the fixed and rotatable members, forming the kinematic support, may confront each other axially, and be biased into engagement by the axial biasing device.
A second aspect of the present invention provides a probe arm for a machine tool, comprising a rotary mechanism as described above, the rotatable member comprising or carrying an arm member on which is located a probe. The probe may, for example, be a toolsetting probe.
Reference should be made to U.S. Pat. No. 5,446,970 for a discussion of the meaning of the terms xe2x80x9ckinematicxe2x80x9d, xe2x80x9ckinematicallyxe2x80x9d and like terms, as used in this specification. These terms encompass not only kinematic supports in which point contacts are provided between the respective pairs of elements on the fixed and rotatable members, but also semi- or quasi-kinematic supports, in which there are small areas or lines of contact between the respective elements.